There is a prevailing need for the detection of explosives, particularly in the aviation industry, but also in other mass transit modes of transportation. More specifically, there exists a need for technology to detect various explosives and hazardous materials that are in liquid or powdered form and also to distinguish hazardous materials, such as homemade explosives, acids, oxidizers, and flammable liquids from benign materials, such as medical liquids, baby formula, beverages, lotions, hygiene products, contact lens solutions and the like.
A material's dielectric constant (permittivity) is determined by measuring its response to a harmonic electric field generated, for example, by a vector network analyzer. Quantifying a material's dielectric constant at selected frequencies or within a frequency range is important for applications relating to optical properties and power transmission. Applications of particular interest include the detection of explosive materials in personnel screening systems (such as millimeter-wave portals used in airports) and the design of explosive simulants that reproduce the optical properties of explosives for the purpose of testing and system validation of various detection systems. Dielectric detection is also used for quality control in chemical, medical, and pharmaceutical applications, such as testing for chemical change or purity or monitoring the curing and aging of concrete.
The dielectric constant of a material can change based on the frequency of applied electromagnetic radiation. Various laboratory techniques have been applied to measure the dielectric constant of materials in radar, microwave, and millimeter-wave frequency bands, including: free-space measurements, performed by transmission and reflection of radiation through a planar slab of material between two transmitting horns; coaxial probes, which measure a reflected signal due to fringing fields from a coaxial line in contact with the material; and waveguide cells, where transmission and reflection through a sample of material loaded into a waveguide or coaxial line is measured. The aforementioned techniques rely on optical effects at the material interfaces to detect the dielectric constant.
Resonant techniques have also been used to measure the dielectric constant of materials. Resonant techniques are different from optical techniques in the sense that the material is incorporated into the electromagnetics of the system. Resonant methods provide the most accurate measurement of the complex dielectric constant of low-absorption materials, i.e., where the imaginary component of the dielectric constant, related to the absorption loss, is small.
Sample configuration is important in resonant systems. Commercial systems based on resonators most often require planar sheets or discs to be inserted into a resonant cavity. This type of configuration is not suited for loose powders or liquids or practical for all materials. However, prior art systems have been developed including fixtures for resonator systems that hold liquids and powders in addition to solids. The dielectric samples have been held in fixtures of particular geometric shape and dimension. Furthermore, the fixtures are integrated into, and form a significant aspect of, the resonant system.
A prior fixture illustrates the idea of measuring the dielectric constant of a sample in a resonant-post system designed for use with electromagnetic radiation at 20-25 GHz in which the resonant post is modified to enclose a small sample. In this configuration, the post is situated between two flat conducting plates and the arrangement is referred to as an “open resonator”. The fixture is a low-loss plastic cylinder, which comprises the post and sample holder. More details of this “open resonator” can be found in the article Weatherall, James C., Barber, Jeffrey, Brauer, Carolyn S., and Barry T. Smith. “Measurement of the reflectivity and absorptivity of liquids, powders, and solids at millimeter wavelengths using dielectric detection by a resonator-post fixture between parallel conducting plates.” Proceedings of SPIE 8019.80190F (2011): 1-8, which is incorporated herein by reference. However, such an open resonator system has many disadvantages. For example, in the open resonator, electromagnetic fields have numerous harmonic modes, which are present in a large spectrum of overlapping modes, and the fields, which are not contained, leak out of the unit, thus radiating the environment. Additionally, the open resonator is calibrated indirectly and is, therefore, difficult to accurately calibrate with reference standards. As a result, there exists a need in the art for a resonator system for measuring dielectric constants of materials that does not suffer from the above-described deficiencies.